The present disclosure relates to semiconductor devices and methods for fabricating the same, and more particularly relates to a semiconductor device in which an underlying barrier metal is formed on an external electrode terminal, and which includes a semiconductor chip flip-chip mounted to a circuit wiring board, and a method for fabricating the same.
In recent years, with progress toward downsizing of, e.g., information communication equipment and office electronic equipment and improvement in the performance thereof, downsizing and an increase in the number of external terminals for input and output are demanded of semiconductor devices mounted in such electronic equipment.
In contrast, with significant progress in semiconductor fabrication processes, progress has been made also in miniaturization of the semiconductor chip structure and an increase in the degree of integration thereof, and thus, a so-called low-k film with a low dielectric constant has tended to be used as an interlayer insulating film.
The physical strength of a low-k film is significantly lower than that of a conventional insulating film, and thus, an active region of a semiconductor chip is susceptible to damage arising from external stress, etc. To prevent this, when semiconductor chips in which progress has been made toward miniaturization and an increase in the degree of integration are to be connected to external circuits, in particular, in flip-chip fashion, attention has been given to fusion bonding using solder bumps.
Features of semiconductor chips having been developed in recent years include flip-chip mounting in which an electrode pad is formed on an active region, and an interlayer insulating film for which a low-k material is used. However, the use of such a mounting method and such a material reduces the reliability of semiconductor chips.
When the material of a semiconductor chip is different from the material of a circuit wiring board on which the semiconductor chip is mounted, displacement between the semiconductor device and the circuit wiring board is often caused due to a difference in coefficient of thermal expansion therebetween. The caused displacement causes stress-induced strains on bumps providing connection between the semiconductor device and the circuit wiring board, and the stress-induced strains result in destruction of the junction interfaces between bumps for flip-chip mounting and electrode pads. To address stress-induced strains arising from thermal expansion, surrounding regions of electrode pads are filled with an underfill material or a sealing resin material (hereinafter together referred to an underfill material).
However, the underfill material contains a filler having a strength which allows mitigation of the effect of stress-induced strains arising from thermal expansion, and a solvent. Therefore, when the shape of a region to be filled with the underfill material is complicated, the region is not uniformly filled with the filler, and thus, is filled with only the solvent. In this case, when the underfill material is cured by heat, the filler is in close contact with bumps at the contact surfaces between the filler and the bumps in the region filled with the filler, thereby forming a structure which is strong enough to withstand strains, etc. In contrast, in the region filled with only the solvent, the solvent is vaporized by heat, and thus, voids or bubbles (hereinafter together referred to as voids) are formed. As a result, cracks are formed from voids formed on the junction interfaces between the bumps and the electrode pads, and the formed cracks reduce the reliability of the semiconductor chip, i.e., the semiconductor device.
To reduce the load on semiconductor chips, a method has been presented in which in order to reduce stress concentration on a connection interface of solder bumps providing physical connection between a semiconductor chip and a circuit wiring board due to the difference in thermal expansion therebetween, an annular resist film is formed around a region on which a solder bump is formed (see, e.g., Japanese Patent Publication No. 2005-268442). When the solder bump is formed, and then, a surrounding region of the solder bump is to be filled with an underfill material, the annular resist film protects a surrounding region of an electrode pad to prevent the underfill material from penetrating the junction interface between the solder bump and the electrode pad.
As described above, a flip-chip method in which a semiconductor chip includes electrode pads formed on an active region, and is connected to an external circuit through external connection terminals called bumps has been employed in general. Furthermore, with significant progress in the semiconductor fabrication process, progress has been made also in miniaturization of the semiconductor device structure and an increase in the degree of integration thereof, and thus, a low-k film with a low dielectric constant has tended to be used as an interlayer insulating film. Thus, in order to reduce thermal stress from external connection terminals to electrode pads, efforts have been made as described in, e.g., Japanese Patent Publication No. 2005-268442.
However, with further downsizing, miniaturization, and an increase in the degree of integration of semiconductor devices, situations are arising where the reliability of semiconductor chips cannot be maintained. Here, in future, there is an urgent need to reduce the load on electrode pads on which bumps are formed, in particular, to reduce the pitch between adjacent bumps, i.e., to provide a technique which can accommodate pitches of, e.g., 50-200 μm.
A semiconductor device is made of silicon (Si), gallium arsenide (GaAs), or any other material, and an inorganic material, such as glass, aramid fibers, or ceramic, and a metal material, such as copper (Cu), are used for a circuit wiring board. Thus, as described above, a material of the semiconductor device is different from a material of the circuit wiring board on which the semiconductor device is mounted.
A process for fabricating a semiconductor device includes a process step of heating a semiconductor substrate at approximately 250-300° C.; a semiconductor chip containing silicon as a main ingredient has a coefficient of thermal expansion of 3 ppm/° C.; and a circuit wiring board containing glass fibers as a main ingredient has a coefficient of thermal expansion of approximately 10 ppm/° C. This causes a difference in the degree of expansion between the semiconductor chip and the circuit wiring board when the semiconductor chip and the circuit wiring board are heated. Here, displacement between the semiconductor device and the circuit wiring board due to the difference in the coefficient of thermal expansion therebetween causes stress-induced strains on bumps providing connection therebetween. The caused stress-induced strains result in destruction of the bumps, leading to poor electrical connection.
In Japanese Patent Publication No. 2005-268442, as illustrated in FIG. 8, a semiconductor chip 101 includes an electrode pad 102 selectively formed on the semiconductor chip 101, an underlying barrier metal (UBM) 110 covering the electrode pad 102, and a solder bump 106 formed on the UBM 110. Here, the UBM 110 is formed such that an outer portion of the UBM 110 is astride the inner edge of an insulating film 109 formed around the electrode pad 102. An annular resist film 126 is formed on the insulating film 109, and the inner end surface of the resist film 126 is in contact with the end surface of the UBM 110 and the end surface of the solder bump 106. As such, the annular resist film 126 is formed outside the solder bump 106 to prevent separation of the UBM 110 due to stress caused in the vicinity of the junction between the solder bump 106 and the electrode pad 102 in a heating and cooling cycle after flip-chip mounting. Specifically, when the annular resist film 126 is formed on a region of the semiconductor chip 101 located around the electrode pad 102, this allows the solder bump 106 to be fixed, and thus, can improve the connection between the bump 106 and the UBM 110.